Immunotherapy with PD-1 or PD-L1 blockade is changing the landscape of cancer therapy, as best evidenced in melanoma with sustained tumor responses in a significant number of patients leading to the first approval of an anti-PD-1 therapy by the FDA. We have been studying response and resistance to this therapy and we are now in the position to propose mechanistic studies aimed at providing a molecular understanding of response, innate and acquired resistance to PD-1 blockade. In the 1st theme I propose to characterize the T cell responses unleashed by blocking PD-1 in patient-derived samples. This will include phenotypic and functional studies in tumor infiltrating lymphocytes (TIL), as well as analyses of the fine antigen specificity of the therapeutic T cells. With the lon term follow up of the initial cohorts of patients treated with the PD-1 blocking antibody pembrolizumab we are now seeing cases of delayed tumor progression after a long period of objective tumor response. We will study the mechanisms of acquired resistance by analyzing changes in T cell function, antigen processing machinery and mutational changes, leading to the loss off neoepitopes due to immune editing. The 2nd theme is centered in research that has been a longstanding theme in my laboratory and that now is becoming a clinical reality. We had hypothesized that targeted therapies could sensitize cancer cells to immunotherapy, and this led my group to work on the clinical development of BRAF inhibitors and study the molecular mechanisms of resistance. But the goal was always to use them in combination with immunotherapy. BRAF inhibitors have a series of properties that make them very good candidates for combination, but it is evident that the translational efforts require adequate preclinical modeling and mechanistic understanding in patients. The 3rd theme builds on my 15 year experience in investigator-initiated cell therapy-based cancer immunotherapy clinical trials. We started with dendritic cell vaccines, both peptide pulsed and genetically modified, and realized that the benefit was restricted to occasional patients with long-term responses. This brought my group to make a big investment in setting up a T cell receptor (TCR) engineered adoptive cell transfer (ACT) program, which is one of the very few in the world. We are now combining TCR engineered ACT with checkpoint blockade therapy and I propose to pursue this research during the funding period. In conclusion, our proposal is based on hypothesis-driven bench-to-bedside-and-back mechanistic studies with the goal of patient-centric advances in tumor immunotherapy for melanoma.